El Presador (Black Ops - Xbox)
rivaLXfactor (Battlefield 3 - PC)
Pwnstar (Battlefield 3 - PC)
Sandy Ravage (Modern Warfare 2 - Xbox)
Xcal (Battlefield 3 - PC)
zzirGrizz (Black Ops II - Xbox)
I'd say Sandy and Grizz were the most difficult opponents, El Presador and Pwnstar the least, with Xfactor and Xcal in the middle (though I'm sure Xcal would've been much more difficult, had we been playing CoD on Xbox)...
...though the most challenging experiences I've had against anyone is playing against a full party of C4B/L3 members